Ethernet network devices commonly utilize data communications media that include multiple full-duplex communications channels. For example, an Ethernet communications medium may include two or four pairs of twisted wire. An Ethernet network device that is compliant with IEEE 802.3ab (1000BASE-T) Gigabit Ethernet standards utilizes a data communications medium that includes 4 pairs of twisted wire. The Gigabit Ethernet network device also employs a full-duplex transmission scheme. Therefore, each of the 4 pairs of twisted wire simultaneously transmit and receive data. However, the transmit and receive signals overlap and may interfere with each other.
Referring now to FIG. 1, first and second exemplary Ethernet network devices 10 and 12, respectively, communicate over a data communications medium with four full-duplex channels 14-A, 14-B, 14-C, and 14-D. For example, the first and second Ethernet network devices 10 and 12, respectively, may be Gigabit Ethernet network devices. Each of the channels 14 at the first and second Ethernet network devices 10 and 12, respectively, are identified as A, B, C, or D and include a transceiver 16 and a hybrid 18. The transceivers 16 independently process transmitted and received data. The hybrids 18 facilitate full-duplex communications over the data communications medium.
Echo is interference between transmitted and received data on an individual channel. Echo may be generated when a near-end transmitted signal is reflected from a transmit path onto a receive path. Echo may also be generated when at least a portion of a transmitted signal on an individual pair of twisted wire is reflected back from the target device. FIG. 1 illustrates both near echo 20 and far echo 22 with respect to the transceiver 16-A-1 on channel A of the first Ethernet network device 10.
Near-end crosstalk (NEXT) is interference between received data on one channel and transmitted data on one or more of the remaining channels of a data communications medium. FIG. 1 illustrates NEXT interference 24-B, 24-C, and 24-D from channels B, C, and D 14-B, 14-C, and 14-D, respectively, that is received at channel A 14-A of the first Ethernet network device 10. Therefore, the received signal at channel A 14-A of the first Ethernet network device 10 potentially includes data transmitted from channel A 14-A of the second Ethernet network device 12, echo 20 and/or 22 from channel A 14-A of the first Ethernet network device 10, and NEXT 24-B, 24-C, and 24-D, respectively, from channels B, C, and D 14-B, 14-C, and 14-D of the first Ethernet network device 10.
Referring now to FIG. 2, a physical layer device 32 of an exemplary Ethernet network device processes data for a full-duplex communications channel of a data communications medium. The physical layer device 32 includes a receive path 34 and a transmit path 36. An input of a first analog filter 38 receives an analog receive signal from the communications channel. The first analog filter 38 filters the analog receive signal and generates a filtered receive signal. An input of an analog-to-digital converter (ADC) 40 receives the filtered receive signal and generates a digital receive signal. A first input of a digital signal processor (DSP) 42 receives the digital receive signal and generates a recovered bit pattern. In an exemplary embodiment, the DSP 42 transmits the recovered bit pattern to a descrambler in a physical coding sublayer (PCS) device in the physical layer device 32.
A second input of the DSP 42 in the receive path 34 receives a scrambled bit pattern from a scrambler in the PCS device. The DSP 42 outputs a digital transmission signal based on the scrambled bit pattern. An input of a digital-to-analog converter (DAC) 44 receives the digital transmission signal and generates an analog transmission signal. An input of a second analog filter 45 receives the analog transmission signal and outputs a filtered transmission signal. For example, the second analog filter 45 may transmit the filtered transmission signal to a line driver in the communications channel.
The input of the first analog filter 38 receives an echo signal. The echo signal is interference from the filtered analog transmission signal. The input of the first analog filter 38 also receives NEXT interference from the other communications channels of the data communications medium. The contribution of echo/NEXT interference may be significant compared to a remotely transmitted signal.
An echo/NEXT cancellation system may be employed to reduce adverse effects caused by echo/NEXT interference at the input of the first analog filter 38. In one approach, multiple analog and/or digital echo/NEXT cancellers are employed to reduce adverse effects from echo/NEXT interference signals in the channel. However, adjusting the operating parameters of multiple echo/NEXT cancellers is very complicated. Also, additional echo/NEXT cancellers require additional clock signals in the channel, which makes clock signal synchronization difficult.